Machines for the above purpose include tentering clamps that have a clamping portion and a roller section carried by a clamp body. The guide roller or rollers of the roller section travel along guide tracks which have an inlet section, an intermediate section, and an outlet section or zone. Each section has two portions, namely one portion for each film web edge. The intermediate section forms treatment zones, and the inlet section and the outlet section extend in parallel to each other, however spaced from each other in the axial direction. The portions of the intermediate section with the treatment zone or zones diverge to form a bridging connection between the inlet section and the outlet section. The guide track portions of the outlet section are spaced from each other in the crosswise direction at a distance larger than the respective spacing of the guide track portions of the inlet section. The tentering clamps are secured to pulling devices referred to herein as tentering chains that travel along the guide tracks in an endless manner. The endless tentering chains travel around a guiding device at the inlet section of the machine and the chains are driven by a drive device such as a wheel at the outlet end of the outlet section.
German Patent Publication DE-AS 1,629,562 (Tsuruta et al.), published on Feb. 4, 1971, relates to a method for the biaxial stretching of film webs made of polyamide. The apparatus used in German Patent Publication 1,629,562 comprises tentering chains in the form of articulated conveyors. These articulated conveyors carry tentering clamps arranged in sequence along the conveyors. Neighboring tentering clamps are interconnected by guide rods that zig-zag back and forth along and between two guide tracks, which guide the tentering chain in the stretching and setting zones. The two guide tracks of each tentering chain are so arranged that with the help of the articulating guide rods, the spacing between the two guide tracks becomes increasingly smaller in the stretching zone and increasingly larger in the setting zone. Tentering chains of this type pose the problem that obtaining a change in the longitudinal stretching ratio or a change in the longitudinal shrinking ratio can be obtained only with a substantial technical effort and expense in that it is necessary to change the position of the two guide tracks of each tentering chain relative to each other. Such positional change is not simple.
U.S. Pat. No. 3,150,433 (Kampf et al.), issued on Sep. 29, 1964, discloses a stretching machine in which the edges of a film web are gripped by tentering clamps which travel along endless revolving guide tracks. The individual tentering clamps are not integrated into tentering chains. Rather, the tentering clamps, also referred to as grippers, are transported individually. For this purpose, each tentering clamp or gripper has a pin that engages a threaded rotating spindle. Two diverging spindles are arranged to slant relative to each other to increase the spacing between the two tentering devices in the diverging treatment section. The rotation of the spindles which are arranged on the left and right side alongside the film web, transport the grippers with a slant relative to the longitudinal direction in the treatment zone. Further, the pitch of the spindle threading increases in the transport direction so that the relative spacing between the grippers is increased, thereby causing a longitudinal stretching of the film web. A machine of this type is limited to a fixed stretching ratio in the longitudinal direction, because the pitch of the threading in the spindle is fixed. Any change in the stretching ratio would require the replacement of these spindles. Another drawback resides in the fact that the production of such spindles is quite involved and the spindles also have a limited length. Spindles that are too long tend to bend unless they are rather bulky. Thus, these spindles can be employed only along a straight path of limited length. A further drawback is seen in the fact that a longitudinal shrinking of the film web is not possible in such an apparatus as disclosed in U.S. Pat. No. 3,150,433.
German Patent Publication DE 3,207,384 (Rottensteiner), published on Sep. 23, 1982, discloses a film web stretching apparatus permitting a biaxial stretching in which the individual tentering clamps are carried and transported by endless tentering chains. Each tentering clamp body carries gear wheels that mesh with corresponding elements in the tentering chains, whereby rotation of these gear wheels causes a displacement of the tentering clamps along the tentering chains. Such displacement increases or decreases the spacing between neighboring clamps for the intended stretching operation. The gear wheels themselves are driven on each tentering clamp by respective drive devices that are controllable. For example, electric motors or controllable gear drives are suitable for this purpose. Thus, the mutual spacing between neighboring tentering clamps along the respective tentering chain is adjustable by an independent, outwardly controllable control member such as the above mentioned electric motor or gear drive. In this type of construction longitudinal force components are distributed onto each individual tentering clamp while the main forces for the movement of the tentering chains are introduced into the respective tentering chain. The tentering chains are not elastic and thus can be dimensioned without difficulties for the load that is caused by the above mentioned longitudinal force components. The apparatus of German Patent DE 3,207,384 permits the simultaneous biaxial stretching of the film webs in a wide range and with controllable stretching ratios and/or shrinking ratios. In fact, it is possible with this known apparatus, to adjust individually different ratios along the length of the treatment section of the apparatus. Further, the known tentering clamps are equipped with a roller guide so that high transport speeds can be realized while still maintaining a quiet and smooth run. However, these advantages are bought with a substantial effort and expense for control components, gear drives, motors, and so forth. The installation of these components on the tentering clamps along the guide track or tracks is frequently not easy to realize, especially because these movable components must travel through a hot environment in the treatment chamber and they must remain fully operational in such hot environment. Another drawback is seen in that the tentering chains are not easily accessible for maintenance work, which may become frequent since these chains run along the guide track with different speeds. Additionally, the treatment of film webs in this context requires not only a precisely controlled stretching, but also a precisely controlled shrinking during the setting operation or treatment. Thus, for the shrinking it is necessary that the spacings between neighboring tentering clamps along the tentering chains must also be precisely controlled during the shrinking of the film web to assure the desired degree or measure of shrinkage.
European Patent Publication 0,291,775 (Langer), published on Nov. 23, 1988, discloses an apparatus for the simultaneous biaxial treatment of film webs. European Patent Publication 0,291,775 corresponds to U.S. Pat. No. 4,890,365 (Langer), issued on Jan. 2, 1990. The edges of the film web are clamped in the Langer apparatus by tentering clamps that are mounted to chain links forming chains running around an endless track. The connection between two successive tentering clamps is established by two chain links journalled to each other. These chain links can be brough in pairs into a lengthwise extending arrangement or into a kinked arrangement and vice versa. This positional change of the chain links is controlled by control rails arranged outside the chain guide track, whereby one of the two chain links is constructed as a bellcrank carrying at a free end of one of its arms, a roller engaging the control rail. Thus, the spacing between two successive tentering clamps can be shortened or lengthened, thereby controlling the film stretching and the film shrinking with a fine ratio. The control features in this known Langer apparatus are quite substantial and leave room for improvement.